Hexafluoroacetone and hexafluoroacetone hydrate are useful as intermediates in the synthesis of various fluorine-containing compounds. For instance, hexafluoroacetone is reacted with various aromatic compounds, and the resulting products are used as a crosslinking agent for rubber or as a monomer for fluorine-containing polyimides. Hexafluoroacetone hydrate is reduced to hexafluoroisopropanol by hydrogen reduction and is used as a material for anesthetics.
For the preparation of hexafluoroacetone, various methods have been known so far, and methods such as the isomerization of hexafluoro-1,2-epoxypropane have been proposed. For example, U.S. Pat. No. 3,321,515 discloses a method of isomerizing hexafluoro-1,2-epoxypropane in the presence of a catalyst such as Al2O3, TiO2, WO2, AlCl3, AlBr3, SnCl4, VoCl3, TiCl4, FeCl3, CuCl2, ZrOCl2 and so on. However, in the method using these catalysts, conversion of the starting material, hexafluoro-1,2-epoxypropane, is not satisfactory, resulting in a low yield of hexafluoroacetone. Improvement in this respect has been strongly desired.
In attempts to increase the conversion of hexafluoro-1,2-epoxypropane, Japanese unexamined patent publication No. 1978-25512 discloses a method of using a fluorinated alumina catalyst; and Japanese unexamined patent publications No. 1983-62130, No. 1983-62131, etc. describe methods of using as a catalyst, fluorinated metal oxides principally comprising fluorinated chromium oxide, fluorinated aluminum oxide, or the like. Although the use of these catalysts enhances the conversion of hexafluoro-1,2-epoxypropane and makes the yield of hexafluoroacetone comparatively high, pentafluoropropionyl fluoride, trifluoroacetyl fluoride, or the like is formed as a by-product. Of these by-products, pentafluoropropionyl fluoride is especially problematic, because the boiling point of pentafluoropropionyl fluoride is very close to that of hexafluoroacetone. Thus, it is difficult to separate pentafluoropropionyl fluoride from hexafluoroacetone by distillation.
As disclosed in Japanese unexamined patent publications No. 1984-204142, No. 1992-10456, etc., hexafluoroacetone is absorbed into water to produce its hydrate, which is then reduced by hydrogenation in the presence of a catalyst at about 70° C. to about 75° C. to be converted into hexafluoroisopropyl alcohol.
However, the obtained hexafluoroacetone usually contains about 2% pentafluoropropionyl fluoride. When the hexafluoroacetone containing such impurities is dissolved in water for hydration, pentafluoropropionyl fluoride is decomposed to produce the corresponding carboxylic acid and hydrogen fluoride (HF), resulting in a solution highly corrosive to metal.
When this solution is used as a material for the preparation of hexafluoroisopropyl alcohol, heavy corrosion of the reactor and deterioration of the catalyst tend to occur. Therefore, acid-free hexafluoroacetone hydrate is strongly desired.
In an attempt to produce acid-free hexafluoroacetone, U.S. Pat. No. 3,544,633, for example, discloses a method of feeding acid-containing hexafluoroacetone to a weakly alkaline salt such as sodium carbonate etc. at a temperature below 160° C. In this method, however, neutralizing acid fluoride such as pentafluoropropionyl fluoride etc. is not easy. Further, the method shows extremely poor workability in operations such as removing the salts formed by the neutralization of HF.